The invention relates to a voltage controlled oscillator provided with a resonant network, an amplifier and a reactive network, all incorporated in an oscillator loop, the reactive network having one or more reactive components whose values depend on a control signal fed to a control input, so that the oscillator frequency can be regulated with said control signal. Such a voltage controlled oscillator is known from the Netherlands Patent Application 8800119.
By presenting a temperature-dependent signal to the control input in the case of such a voltage controlled oscillator, which is usually termed a VCO or, in the case where the resonant network is a quartz crystal, a VCXO, it is also possible to compensate for the temperature variation of the oscillator frequency. Such oscillators are known as TCOs or, in the case where the resonant network is a quartz crystal, as TCXOs (temperature compensated crystal oscillator).
The variable reactive component or variable reactance, for example formed by a varactor diode, associated with the reactive network, may be incorporated optionally in combination with other reactive components in series with or parallel to the resonant network, but may also be incorporated at another position in the oscillator loop. The component does not necessarily have to be passive and other active reactance circuits are also suitable. In the latter case, the controllability can be obtained by varying, for example, a gain factor or a resistor. The implementation will always have to be such that the frequency, for which the imaginary part of the gain A.sub.L round the oscillator loop becomes equal to zero, can be influenced by the value of the regulable reactive component. The reason is that it is only for this value of the imaginary part that the first Barkhausen oscillation condition, i.e. Im{A.sub.L }=0, can be fulfilled.
The values of the reactive components in the reactive network are, in general, not accurately known, dependent upon temperature and nonlinear. In order to cause the oscillator frequency to be accurately determined by the resonant network in the quiescent situation, i.e. in the absence of a control signal, it is necessary for the imaginary part of the loop gain introduced by the oscillator amplifier and the reactive components of the reactive network to be made equal to zero. Normally this is done by means of an adjustment of the component values of the reactive components in the reactive network. Said component values are, however, dependent on the temperature and their temperature coefficients are subject to spread. In addition, the relationship between component value and control signal is, in general, not linear and this nonlinearity is also subject to spread. In order to be able to control the oscillator frequency in a well defined way, for example in order to achieve a linear relationship between control signal and frequency variation, as is desirable in VC(X)0s or in order to compensate for the temperature variation of the oscillator frequency as is desirable in TC(X)0s, it is first necessary to carry out measurements in order to determine the relationships between control signal and frequency, or between temperature and frequency. Only if these relationships, which have to be measured separately for every oscillator owing to the component spread, are known, is it possible to design correction networks so that the desired linearity or temperature independence is obtained. These procedures are time-consuming and therefore expensive.
In the voltage controlled oscillator known from the abovementioned patent application, the effect of inaccuracy, dependence on temperature and nonlinearity of the reactive components are substantially eliminated. For this purpose, the said reactive network in the oscillator, which is in this case a phase shifting network, is provided with a separate control loop having a phase detector which is so designed that the sensitivity of the oscillator is accurate and constant even with variation in temperature. For this purpose, a phase shift which is directly proportional to the control signal is impressed on the phase shifting network by means of the phase detector. As a result of this, the oscillator frequency, and its variation with the voltage or current of the control signal, is completely determined by the resonant network.
A problem in this connection is, however, that the quality factor of the resonant network plays a role in said variation and also that said variation is nonlinear. For this reason an accurate control of the quality factor is required in manufacturing the resonant network to realise an accurate voltage controlled or temperature compensated (crystal) oscillator in accordance with this principle. The control of the reproducibility of said quality factor is, in general, extremely difficult.
Depending on the implementation of the phase shifting network of this known voltage controlled oscillator, a second problem may arise. The real part of the small-signal loop gain may vary considerably as a result of alteration of the phase shift. For oscillation, the real part of the gain round the loop must, according to the second Barkhausen condition, always be greater than unity. The amplifier incorporated in the oscillator loop must therefore be designed in a manner such that this second condition is fulfilled for all the required phase shifts. In practice it is known that the large loop gain required for this is at the expense of the short-term stability.